Such heads are utilized essentially in telecommunications for providing optical fiber transmission networks. Major research laboratories have performed numerous experiments on such networks. The experiments have been particularly directed to networks using coherent detection and they have shown that such networks have various advantageous over networks using direct detection:
the energy budget is improved thus making it possible to increase the range of point-to-point links, or the number of stations connected to a network; and
multiplexing can be performed at closer wavelength intervals, thereby making it possible to increase the transmission capacity of a fiber or a network.
In order to obtain such advantages, the optoelectronic sources used in such networks must be high performance sources. In particular, they must have very narrow spectrum line widths. That is why these sources are currently constituted by lasers having distributed resonators, known as distributed feedback (DFB) lasers. Unfortunately, such lasers are detrimentally sensitive to interfering optical reflections which are difficult to avoid, e.g. backscattering in the fiber and reflections from non-optimized connections. The optical feedback from such reflections can disturb the essential characteristics of the source, such as monochromaticity, relative intensity noise (RIN), etc. . . . , and this can penalize the system or even prevent it from operating properly (asymptotic error rate greater than 10.sup.-9).
One known proposal for avoiding these drawbacks consists in integrating an optical isolator in the head in order to stop the light due to interfering reflections.
A first prior optical head using this disposition includes certain items in common with a head of the present invention, and these items are as follows:
a housing having a longitudinal direction with a front end and a rear end;
said semiconductor laser disposed inside said housing in order to emit a beam of useful light forwards along said longitudinal direction, said beam having a plane of polarization which is initially oriented along the plane of polarization of the laser;
a rotator disposed in said housing in front of the laser for rotating the plane of polarization of said beam through about 45.degree. in one direction so as to orient said plane along an outlet plane of polarization;
a focusing lens disposed inside said housing in front of said laser for injecting said beam into the inlet of a light guide; and
a polarizer disposed between said rotator and said guide to be coupled in order to selectively pass light which is polarized along said outlet plane of polarization, thereby allowing said useful light to pass and polarizing any interfering light travelling in the reverse direction along said outlet plane of polarization so that said rotator causes the plane of polarization of said interfering light to rotate through about 45.degree. in said direction so that said light reaches said laser with a plane of polarization which is substantially perpendicular to the said plane of polarization of the laser.
Prior heads of this type are described in the following articles:
"Distributed feedback laser diode (DFB-LD) to single-mode fiber coupling module with optical isolator for high bit rate modulation", by Toshihiko Sugie and Masatoshi Saruwatari, in Journal of Lightwave Technology, Vol. LT-4, No. 2, February 1986; and
"Low-noise LD module with an optical isolator using a highly bi-substituted garnet film", by K. Matsuda, H. Minemoto, K. Toda, O. Kamada, S. Ishizuka, in Electronics Letters, Feb. 26, 1987, Vol. 23, No. 5.
A drawback of these prior heads is that they are constituted by an optical system having four elements (first lens-rotator-second lens-polarizer) or having three elements (lens-rotator-polarizer) inserted between the laser and the fiber. This gives rise to major difficulties in implementation (positioning and adjusting these various parts in order to ensure minimum insertion loss and sufficient isolation).
Another drawback is that the numerous interfaces encountered by the light in the optical head tend to increase the fraction of the light which is reflected into the laser.
The aims of the present invention include, in particular, simplifying the construction and adjustment of an optical head having an integrated isolator, and/or reducing the bulk of said head together with the number of interfaces that are liable to give rise to interfering back reflection.